The company took the drastic, and totally legal, action against David Houghton and girlfriend Abby Simpson in a row over an unpaid bill.

A British Gas engineer gained access to the two-bedroomed flat in Willesden Green, London, with the help of a locksmith, and swapped their meter for a pay-as-you-go version.

The measure is standard practice when customers continually fail to pay their bills, but Mr Houghton and Miss Simpson are customers of rival energy provider EDF.

Proper checks would have revealed the couple did not owe a penny.

But from 2005 the British Gas computer system wrongly began to bill the couple with demands rising from £90 to £900.

Mr Houghton dealt with numerous threats of legal action and visits from the bailiffs, before a personal apology from the energy giant’s managing director, Phil Bentley, convinced him that his troubles were over.

But when the couple went on a long weekend to New York in June 2007, they returned home to find British Gas employees had picked the locks on their front and internal door and installed a new meter.

They then left a note informing the couple of what had happened. Investigators have since worked out that it was the occupants of the next-door flat who owed the money to British Gas.

Mr Houghton said: “I am totally disgusted and bewildered by their behaviour. It felt so intrusive that they had been in our flat and could have gone into every room if they’d wanted to. “I spoke to manager after manager at their call centres and each time was promised the problem had been sorted out.”

A spokesman for the company claimed last night that the Gas Act allows workers to break into a customer’s home to change the meter, providing he or she has been warned in advance. However, Mr Houghton claims he was never notified.